A fuel vapor treatment apparatus prevents fuel vapor developed in a fuel tank from being dissipated to the atmosphere and introduces the fuel vapor into a canister. The canister accommodates an absorbing material to adsorb the fuel vapor temporarily. When an internal combustion engine is operated, the adsorbed fuel vapor is desorbed from the absorbing material by negative pressure developed in an intake pipe and is purged into the intake pipe of the internal combustion engine via a purge pipe. In this manner, when the fuel vapor is desorbed from the adsorbing material, the adsorbing capacity of the adsorbing material is recovered.
When the fuel vapor is being purged, the air-fuel ratio of an air-fuel mixture introduced into the internal combustion engine needs to be controlled to a state close to a target air-fuel ratio (generally, stoichiometric air-fuel ratio). JP-A 7-269419 shows a fuel vapor treatment system in which an air-fuel ratio sensor for actually measuring an air-fuel ratio is provided in the exhaust pipe of the internal combustion engine. A feedback control is performed on the basis of the deviation from the target air-fuel ratio of an air-fuel ratio measured by the air-fuel sensor to control a fuel injection quantity to bring the air-fuel ratio of the air-fuel mixture introduced into the internal combustion engine to the target air-fuel ratio.
In this system, the state of concentration of the fuel vapor of an air-fuel mixture containing fuel vapor purged from a canister is determined on the basis of such a deviation from the target air-fuel ratio of an air-fuel ratio as is measured by the air-fuel sensor. Here, the concentration of fuel vapor is a kind of state of the fuel. A fuel injection quantity is controlled on the basis of the concentration of fuel vapor (that is, state of fuel) to bring the air-fuel ratio to the target air-fuel ratio.
In a case that an air-fuel ratio is actually measured by the air-fuel sensor and the deviation from the target air-fuel ratio of the measured air-fuel ratio is fed back to determine a fuel injection quantity, if the fuel vapor is not purged, the fuel injection quantity cannot be determined.
Hence, it is necessary to bring a purge ratio to a small value not to cause large air-fuel variations when the purging of the fuel vapor is started and then to increase the purge ratio gradually. Moreover, also when the purging of the fuel vapor is interrupted and then is started again, similarly, it is necessary to set an initial purge ratio smaller and then to increase the purging ratio gradually. For this reason, there is presented a problem that the amount of purge cannot be sufficiently increased.